For the purposes of the description of the invention, “flow sensor” means the essentially mechanical portion of an electromagnetic flowmeter without the coil-current-generating circuit. Such electromagnetic flow sensors, as is well known, use Faraday's law of induction to produce a measurement voltage.
In DE-A 20 40 682, U.S. Pat. No. 4,641,537, U.S. Pat. No. 4,774,844, or WO-A 91/11731, different implementations of electromagnetic flow sensors are described, which serve to measure an electrically conductive fluid flowing in a pipe. The flow sensors shown each comprise:                a flow tube designed to be installed in the pipe for conducting the fluid, the flow tube being electrically nonconductive at least on a fluid-contacting inner surface;        an electrode arrangement comprising at least two measuring electrodes disposed on the flow tube for picking up a voltage induced in the fluid; and        a magnetic field system likewise disposed on the flow tube and comprising                    at least a first and a second saddle-shaped field coil for producing a magnetic field which in operation cuts the fluid essentially transversely, particularly at right angles, to a longitudinal axis of the flow tube, and            a ferromagnetic first pole piece for the first field coil and a ferromagnetic second pole piece for the second field coil for directing the magnetic field toward the fluid.                        
While the respective magnetic field systems of the flow sensors disclosed in WO-A 91/11731 have two ferromagnetic return paths for controlling the magnetic field distribution which are disposed at the sides of the field coils and are magnetically coupled to the pole pieces, and which extend annularly and parallel to each other around the flow tube, each of the flow sensors in DE-A 20 40 682, U.S. Pat. No. 4,641,537, and U.S. Pat. No. 4,774,844 is shown having two centrally disposed ferromagnetic return paths magnetically coupled to the pole pieces, these centrally extending return paths being implemented as comparatively thin sheet-metal strips.
Furthermore, at least in the flow sensor shown in U.S. Pat. No. 4,641,537, each of the pole pieces is connected with the return paths by two, likewise comparatively thin, ferromagnetic coupling elements, each of which has a coil-core- and yoke-forming cover segment. These cover segments at least partially cover respective first and second winding sections of the field coils which extend parallel to each other and to the longitudinal axis of the flow tube.
The advantages of such magnetic field systems using saddle-shaped field coils in conjunction with predominantly thin-walled elements controlling the magnetic field distribution outside the flow tube, as is also apparent from U.S. Pat. No. 4,641,537, lie in the comparatively low material expenditure and in their high efficiency. Consequently, such flow sensors, apart from being inexpensive to manufacture provide high measurement accuracy along with a comparatively great operable flow range. Furthermore, the flow sensors can be of a very compact design, at least in the radial direction.
Investigations of such flow sensors have shown, however, that the above-mentioned advantageous properties are found primarily in flow sensors with flow tubes of comparatively small nominal diameter. In the case of pipes with great nominal diameters, and thus at correspondingly great diameters of the flow tube, the necessary measurement accuracy or the desired sensitivity, as also discussed in U.S. Pat. No. 5,540,103, for example, could be achieved with conventional flow sensors using saddle-shaped field coils only by increased material expenditure, particularly by an increased amount of copper for the field coils. In addition it turned out that at great nominal diameters, where the ratio of nominal diameter to flow-tube length is generally greater than for small nominal diameters, the spread of the magnetic field in the axial direction may no longer be negligible.
To reduce variations in measuring sensitivity which may be associated with a spreading of the magnetic field to the connected pipe as frequently occurs with great nominal diameters, many manufacturers recommend grounding measures, for example, which the user is to take when installing the flow sensor. For example, grounding washers are commonly used between the flow sensor and the connected pipe. Another possibility of reducing such dependences of the measuring sensitivity on the mounting conditions is to make the flow tube, but at least the nonconductive portion of the same, sufficiently long in the axial direction and/or, as also indicated in U.S. Pat. No. 5,540,103, for example, to use cylindrical field coils, thus making the field coils and pole pieces comparatively short in the axial direction, albeit at the expense of operable flow range.